Oxygen delignification of paper pulp at medium consistency (i.e., 6-8%) has become increasingly popular since it has been found to be particularly advantageous in many applications. In a typical medium consistency oxygen delignification system, it is necessary to provide a mixer (such as MC.RTM. mixer sold by Kamyr, Inc. of Glens Falls, N.Y.) that is capable of effectively dispersing oxygen into the pulp slurry. The slurry is then subseqently pumped to additional treatment stages, such as additional bleaching stages, pulp storage, a washing stage, or the like.
According to the present invention, a method and apparatus for effecting oxygen delignification of paper pulp is provided that reduces the capital cost of the system, while still providing for effective treatment. These desirable results are accomplished according to the invention by eliminating the mixing that has been considered necessary in order to properly disperse oxygen gas into the pulp, instead providing for the introduction of the oxygen gas into the discharge of a fluidizing centrifugal pump for pumping the pulp slurry.
According to one aspect of the present invention, an apparatus for facilitating oxygen delignification of a medium consistency finely comminuted cellulosic fibrous material slurry (paper pulp) is provided. The apparatus includes a fluidizing centrifugal pump having a housing with a pulp inlet and a pulp outlet, and an impeller disposed within the housing for pumping pulp from the inlet to the outlet. The apparatus further includes means for introducing oxygen gas into the pulp as it is passing out the pulp outlet. The oxygen gas introduction means preferably comprises a plate (e.g. a shear plate) affixed to the housing outlet, and having a central opening substantially concentric with the pulp outlet. One or more generally radially extending bores are provide in the plate, each at a first end thereof in communication with the central opening, and at the other end connected up to a source of oxygen gas under pressure. In order to introduce the oxygen in the form of very small bubbles, so that it is more evenly dispersed in the pulp, a tube may be provided in the bores, the tube having an oxygen permeable portion (such as a porous stone or sintered metal) extending from the bores through the central opening. The fluidizing centrifugal pump typically has a vacuum system associated therewith for degassing pulp during pumping, and the pulp inlet is connected up to a pulp reservoir vessel such as a stand pipe.
According to another aspect of the present invention a method is provided for the oxygen delignification of medium consistency (i.e., about 6-18%) pulp. The method comprises the steps of:
(a) adding caustic to the slurry; PA1 (b) simultaneously fluidizing and degassing the slurry while centrifugally pumping the slurry; and PA1 (c) immediately after step (b), adding oxygen gas to the slurry, to effect oxygen delignification of the slurry material. Step (c) is practiced by introducing the oxygen gas through a shear plate in the discharge of a fluidizing centrifugal pump (which effects step (b)) either directly into the pulp or through an oxygen permeable material which causes the oxygen to be introduced in the form of small bubbles.
It is the primary object of the invention to provide for the simple, yet effective, oxygen delignification of pulp at medium consistencies. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.